The present invention relates generally to the fabrication of stable conjugated polymer electrochemical devices and, more particularly, to the development of stable conjugated polymer electrochromic devices incorporating ionic liquids. Conjugated polymers having electroactivity and electrochroism have also been electrochemically synthesized using ionic liquids as electrolytes as well as by chemical methods, and electrochromic devices fabricated therefrom.
Electrochromic devices, such as displays and smart windows, are fabricated from electrochromic materials that change color in a persistent but reversible manner as the result of an electrochemical reaction. These devices find applications in a wide range of areas; as examples: in the optical and communication industry, as optical information displays and storage media; in the building industry, as architectural glazed windows for energy control and glare-reduction system for offices; in the automobile industry, as anti-glare rear-view mirrors and sun roofs for cars; in the armed services, as protective eyewear for soldiers and controllable canopies for aircrafts; and in daily life, as consumer sunglasses. Electrochromic rear-view mirrors for automobiles have already achieved considerable commercial success. Advantages of electrochromic devices (ED) over liquid crystalline displays (LCD) include transparency and flexibility. In contrast to LCDs, EDs do not polarize light; thus, ED images can be observed for over a large angular distribution.
The most successful and frequently used electrochromic materials to date are inorganic compounds such as WO3, MoO3 and V2O5. These materials are expensive to process which inevitably limits their commercial and industrial applications. Therefore, the use of conjugated polymers as electrochromic materials has become an important research area. In addition to low-cost, conjugated polymers have other advantages over inorganic compounds, such as good processibility for large-area applications, and color selectivity for varying the coloration of chosen regions.
To fabricate electrochromic devices, conjugated polymers are coated on conductive, transparent-glass electrodes such as indium tin oxide. The two principal methods for polymer coating are chemical and electrochemical. Electrochemical processes possess the following advantages when compared to chemical methods: (a) polymerization, doping and processing take place simultaneously when electrochemical methods are employed, while for chemical processing, the initial polymer synthesis must be followed by doping and subsequent processing; (b) polymer thickness is more readily controlled by monitoring the charge consumed; (c) polymer films produced using electrochemical processes often exhibit higher conductivity; and (d) polymer properties are more readily varied by changing dopants.
Generally, the exclusion of moisture/water is of critical importance during the fabrication of solid-state electrochemical devices to ensure the high performance and long-term operation of the devices. Moisture/water may be introduced at any time during the fabrication process of electrochemical devices, including the preparation of polymer electrodes, the preparation of electrolytes and the assembly of the devices. Electrochemical synthesis and deposition of conjugated polymers can be carried out either in aqueous or non-aqueous electrolytes. If aqueous electrolytes are employed, the resulting polymers must be subjected to drying processes, preferably under vacuum, prior to use. For the frequently studied electrochromic material polyaniline, other problems have been identified relating to the electrochemical synthesis of polyaniline in aqueous electrolyte systems; for example, degradation of polyaniline during polarization (See, e.g., T. Kobayashi et al., J. Electroanal Chem. 177, 273 (1984)); and decrease in electrochemical activity due to nucleophilic attack on the polymer by OHxe2x88x92 (See, e.g., T. Osaka et al., J. Electrochem. Soc. 136, 306 (1989)).
As a result, synthesis of conjugated polymers in non-aqueous electrolytes has generated extensive attention (See, e.g., T. Osaka, et al., J. Electrochem. Soc., 1991, 138 (10), 2853. and K. Yamada et al., J. Electroanal. Chem., 1995, 394, 71.). Evaporation of the organic solvents employed in the synthesis and the absorption of moisture to varying degrees by these solvents affects the long-term operation of the subsequently fabricated devices and makes it desirable to prepare these conjugated polymers in a dry glove box.
Additionally, moisture may be introduced during the preparation of the electrolytes used for device fabrication and the final assembly of the devices in air. The operating voltage and lifetime of the resulting devices are affected by the water content of the electrolytes resulting from the miscibility with water of the organic solvents employed. The evaporation of the organic solvents also limits the long-term operation of the devices. Thus, lack of volatility and lack of miscibility with water are important properties of electrolytes for the fabrication of durable and stable electrochemical devices.
Ionic liquids have received considerable attention as electrolytes in various electrochemical devices (See, e.g., A. B. McEwen et al., Electrochemical Capacitors II, F. M. Delnick et al., Editors, PV 96-25, p.313; V. R. Koch et al., J. Electrochem. Soc. 142, L116 (1995); V. R. Koch et al., J. Electrochem. Soc. 143, 788 (1996); J. S. Wilkes, and M. J. Zaworotko, J. Chem. Soc. Commun. p.965 (1992); R. T. Carlin et al., J. Electrochem. Soc. 141, L73 (1994); P. Bonhxc3x4te et al., Inorg. Chem. 35, 1168 (1996); and N. Papageorgiou et al., J. Electrochem. Soc. 143, 3099 (1996)). However, the use of ionic liquids as electrolytes for the fabrication and development of conjugated polymer electrochemical devices has not been previously addressed, and the study of ionic liquids as electrolytes in conjugated polymer electrochemistry is limited. Several electroactive polymers (polypyrrole, polythiophene and polyaniline) have been prepared in ionic liquids and the resulting polymer films have exhibited electroactivity in these ionic liquids (See, e.g., P. G. Pickup and R. A. Osteryoung, J. Am. Chem. Soc., 106, 2294 (1984); P. G. Pickup and R. A. Osteryoung, J. Electroanal Soc. 195, 271 (1985); L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc. 134, 2787 (1987); L. Janiszewska and R. A. Osteryoung, J. Electrochem. Soc. 135, 116 (1988); J. Tang and R. A. Osteryoung, Synth. Met 45, 1 (1991); J. Tang and R. A. Osteryoung, Synth. Met. 44, 307 (1991); and J. Tang et al., J. Phys. Chem. 96, 3531 (1992)). The research described in these papers was performed using aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl3-IMCl) as an electrolyte. U.S. Pat. No. 5,827,602 which issued to V. R. Koch et al. asserts that a disadvantage of these ionic liquids and a problem with any ionic liquid containing a strong Lewis acid such as AlCl3, is the liberation of toxic gases when the ionic liquid is exposed to moisture. Additionally, the high reactivity of the Lewis acids used as a component of the ionic liquids limits the types of organic and inorganic compounds which are stable in these media. Typically, room-temperature ionic liquids containing Lewis acids decompose below 150xc2x0 C.
Accordingly, it is an object of the present invention to provide a method for producing durable and stable conjugated polymer electrochromic devices using ionic liquids as electrolytes.
Another object of the present invention is to provide an electrochemical synthesis of conjugated polymers in ionic liquids.
Yet another object of the invention is to achieve electroactivity and electrochroism in conjugated polymers in contact with ionic liquids.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the method for preparing an electroactive conjugated polymer includes the steps of: immersing a pair of electrodes in an ionic liquid solution containing the monomeric form of the conjugated polymer; and applying a voltage between the electrodes in the pair of electrodes until a desired quantity of conjugated polymer which is stable in the presence of the ionic liquid is deposited on one of the electrodes in the pair of electrodes.
Preferably, the voltage applied between the electrodes in the pair of electrodes can be a constant voltage, a voltage which establishes a constant current between the electrodes or a voltage cycled at a chosen scan rate.
It is preferred that the ionic liquid is stable in the presence of water.
In another aspect of the present invention in accordance with its objects and purposes, the electrochromic device hereof includes: a first electrically conducting substrate; a second electrically conducting substrate spaced apart from the first substrate; a first conjugated polymer deposited onto the surface of the first substrate facing the second substrate; a second conjugated polymer deposited onto the surface of the second substrate facing said first substrate; an ionic liquid electrolyte disposed between and in electrical contact with the first conjugated polymer and the said second conjugated polymer; and an electrical power supply for applying a voltage between the first substrate and the second substrate.
Preferably, the first conjugated polymer includes an anodically coloring conjugated polymer and the second conjugated polymer includes a cathodically coloring conjugated polymer.
It is preferred that the first substrate and the second substrate include conductive and optically transparent substrates.
It is also preferred that the deposited conjugate polymer is stable in the presence of the ionic liquid.
Preferably also, the ionic liquid is stable in the presence of water.
Benefits and advantages of the present invention include the preparation of conjugated polymers in ionic liquids in the absence of moisture and oxygen so that no drying is required before the polymers are incorporated into electrochromic devices. Moreover, there is less degradation over time of the resulting polymers which results in longer lifetimes therefor. Use of ionic liquids permits high-performance electrochromic devices having lower operating voltages and increased speed for coloration changes to be fabricated.